Metallic porous plates

ABSTRACT

Porous plates made by vacuum evaporation of metal at relatively low vacuum levels. Such plates can be removed from the substrate for use. The pores in the porous structure so produced may be enlarged by etching.

United States Patent 151 3,692,087 Eberts 1451 Sept. 19, 1972 [54]METALLIC POROUS PLATES 3,401,736 9/1968 lmogawa .;.....l64/46 2,409,29510/1946 Marvin et a1 ..1 17/107 X [721 Invent Ebem Fammgham' 3,022,1872/1962 Eyraud et a1 ..117/107 x Mass 3,355,320 11/1967 Spriggs et al..l17/l07 X [73] Assignee: Norton Company, worchester, 3,390,026 6/ 1968Cerych et a1 ..1 17/107 X Mass' Primary Examiner-Alfred L. Leavitt 1 1Flledl March 1969 Assistant Examiner-Kenneth P. Glynn [211 App. No:807,814 Attorney/Oliver W. Hayes and Jerry Cohen 57 ABSTRACT 52 us. c1...164/46, 117/107, l48/6.14 Porous plates made by vacuum evaporation of[51] f Cl-WBnd Z3/OOC23C 13/O0'C23c 13/02 at relatively low vacuumlevels. Such plates can be Fleld of Search 143/6-14; removed from thesubstrate for use. The pores in the 164/46 porous structure so producedmay be enlarged by etching. [56] References Cl ed 3 Claims, 9 DrawingFigures UNITED STATES PATENTS 3,181,209 5/1965 $511111, Jr. .J "16174 34'wwwK/- 13 1 i I V 20 1 1,

PKTENTEDSEP 1 9 m2 SHEET 2 BF 4 INVENTOR ROBERT E. EBERTS Fig. 7.

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ATTORNEY;

PNENTED SE? 19 m2 SHEET U 0F 4 ATTORNEY r .1 METALLIC POROUS PLATES Thepresent invention relates to production of fluid (gas and liquid)permeable, porous plate elements for use in filtering, sorption, reverseosmosis, back-up plates, fuel cell electrodes and like applications.

Porous plate elements of this character have been the subject ofintensive research and development for several years. Currently usedmaterials are made of organic materials, felted or woven glass fibersand sintered or compacted metal powders and fibers. Each type hascertain limitations and restrictions. For instance, organic materialsare likely to compress under pressure and thus lose their permeability;they also must be used at relatively low pressures toprevent bursting.It is difficult to make porous glass fiber materials with sufficientlysmall pore sizes and in sufficiently thin sections. The uniformity ofpore size is difficult to control in metal powder compacts and thicknessis generally greater than desired.

It is the object of the present invention to provide an improvedpermeable porous plate element of this character having pore size lessthan 0.1 X meter (0.1 micron) with minimum sheet thickness consistentwith high permeability andhigh resistance to temperature, corrosion andpressure.

It is a further object of the invention to provide such an element withcontrolled pore size within the desired low range of pore size whilemaintaining a useful, desirable level of permeability.

It is a further object of the invention to provide a method of readilymanufacturing such plates.

' Other objects, features and advantages of the invention will in partbe explained in, and will in part be obvious from thefollowingdescription of the invention.

GENERAL DESCRIPTION The invention is based on the recognition that theconditions which give rise to very poor metal coatings in vacuum coatingpractice or stray deposits of powdery or porous coatings on vacuumcoating chamber walls can bedefined and adjusted to produce porouselements of the character described above. To test this hypothesis, avacuum coater was deliberately run under several sets of conditionsamounting to poor vacuum coating practice and it was observed that metalcoatings could be produced on a substrate-either adherent to or easilyremovable from the substrate, as desiredwhich would exhibit much lowerpore size than conventional plates made from metal powder compacts.

The invention includes the further concept that certainmetals arereadily oxidized and in use as porous plates will tend to build up heavyoxide layers if some fluid permeating the plate is capable of enteringinto oxidation reaction with the metal. This tendency is used toadvantage to control pore size by leaching the pores to remove oxide andre-oxidizingand releaching as necessary to achieve a desired size on acontrollable basis.

The invention encompasses within the term metal the use of any metal,metal compound, metalloid, group of metals or alloys that can be vacuumevaporated to produce a deposit having metal-like characteristics ofsolvent heat and abrasion resistance and strength and hardness comparedto, say, plastics. By simultaneously evaporating more than one metal, analloy plate can be made.

The term pore size as. used herein refers to a minimum limitingdimension which is width in the case of a rectangular section poreanddiameter in the case of a circular cross-section pore. A pore is apassage in a plate running through the thickness dimension of the plateand may be formed by a series of interconnected passages. Plate includesplates which are flat orcurved, supported or unsupported, and are usedwith or without additional layers or coatings adhered thereto.

SPECIFIC DESCRIPTION .mitted to. the chamberfrom a cylinder 15 as tomaintain the vacuum at the desired level. Valve 16 to the pumping systemcan also be used to regulate the vacuum in the chamber. The metal 20 tobe evaporated is contained in a boat or crucible 222 which is heated bya resistanceor induction heater 24. The substrate material30 can bedisposed in the chamber and backed with cooling coils 32 through whichwater or gas can be passed and with an electrical resistance heater 34.A thermocouple 36 is attachedto the substrate for monitoring itstemperatures. The substrate 30 is shown mounted on an adjustablecarriage 38 so that its distancefrom the source can be controlled. Whilethe substratehere is shown as a flat plate, it can be rany shape.Depending on the final size and shape desired, the substrate could be arotating :drum, a concave curved surface, orany other form familiar tothose skilled in the art of vacuum coating. The deposited coating isindicated at 40.

FIG. 2 is a schematic of the final porous plate. This I shows theporosity to be due to fine microcracks that run through the thickness ofthe sheet. To some extent these microcracks appear similar' to grainboundaries when looking at the surface of the sample.

FIG. 3 shows a schematic cross-section of the porous plate. As can beseen, the microcrack pores run through the sheet. By changing thedeposition parameters, a dual structure material can be prepared. Thearrows 41 indicate fluid flow through the plate.

FIG. 4 shows a schematic crosssection of a plate that contains pores inall directions. Such a structure can be obtained by periodicallydiscontinuing the deposition.

FIG. '5 shows a microporous evaporated plate 40 which has been depositedupon a porous sintered metal plate 50 and upon which an organic (e.g.,cellulose acetate) reverse osmosis membrane 60 has been placed.

FIG. 6 is a schematic of the permeability test apparatus used. A inchspecimen disk of a sheet 40 was cut and mounted in a vacuum coupling 76.The actual test area is V4. inch diameter. A vacuum was pulled on and Kspecific permeability q fluid flow rate H viscosity of fluid Across-section area AP= pressure differential L thickness I Test resultsare given for air. However, the plates produced were likewise permeableto water.

FIGS. 7-9 are actual electron micrographs of the surface of the porousmembranes. These show the type of pore obtained. The magnification is15,020 times for FIG. 7, 31,700 times for FIG. 8 and 43,500 times forFIG. 9. The light side of the sheet is viewed in FIG. 9 and the darkside (towards the substrate) in FIGS. 7-8.

EXAMPLE 1 A 4-foot diameter vacuum chamber, with more than adequatepumping capacity was used. A 0.020 inch thick, 1 square foot sheet ofbrass was used as a substrate; this was located 27 inches above thecrucible. The aluminum to be evaporated was contained in a four inchdiameter dish shaped graphite crucible which was induction heated. Thedeposition was carried out for 24 minutes at a vacuum of X 10' torr witha source temperature of l,235 C. and a substrate temperature of 200 C.After cooling the system and venting to air, the substrate and depositwere removed. The plate was removed from the substrate by flexing thebrass. The one square foot porous aluminum sheet was 0.0015 inch thick.In the case of the sheet produced in this run, a flow rate of 4cc/minute was obtained.

EXAMPLE 2 A 9 X 12 inch sheet of 60 mil stainless steel was used as thesubstrate. The sheet was located 27 inches above the graphite crucible.Copper was evaporated at a temperature of 1,420 C. under a vacuum of 2 X10* torr for 47 minutes. The 0.0035 inch thick porous sheet was readilyremoved from the substrate. A flow rate of 5.5 cc/minute was obtained inthe test.

EXAMPLE 3 The 5 X 12 X 0.017 inch brass sheet substrate was placed 27inches from the aluminum evaporation source, which was at 1,235 C. Theevaporation was carried out at about 1 X 10' torr for 4 minutes. The0.001 inch sheet recovered had a flow rate of about 5 cc/minute. Thissheet was soaked in concentrated nitric acid for an hour, rinsed, dried,and retested. The flow rate had increased 2% times.

It was found important that the surface of the substrate be very smoothand scratch-free- Otherwise, it was extremely difficult to remove thedeposited porous sheet. The source to substrate distance can be variedwidely, depending'upon the permeability desired. The shorter thedistance, the more rapid the deposition; the greater the distance, theeasierto maintain substrate temperature. The deposition time can bevaried from a few minutes to a few hours. Some apparently very porous,permeable films were made with only a 3-4 minute run time; these were sothin and fragile that they could not be handled without breaking. Ofcourse,

per se, if deposited directly on the desired large pored back-up plate.The evaporation pressure can be maintained from 1 X 10'? torr to 50 X10' torr.

For obtaining suitable porous sheets the controlling factors are:

Y 1. Substrate temperature between about C. to about one-half themelting point of the metal; 2. Source temperature controls theevaporation rate of the particular metal;

3. Deposition time controls the thickness of the plate;

4. Vacuum effects the evaporation rate and the nature of the deposits.At pressures much under 1 X 10' torr a non-porous plate is obtained;.atpressures over 50 X 10', torr a powdery deposit is expected;

5. Substrate surface if polished and without scratches, plate can beremoved.

6. Substrate distance from source It must be understood that there isinteraction between the above factors. Depending upon the desired resultthese can be varied to give more or less permeability, thicker orthinner sheets, adherent or non-adherent sheets. Those skilled in theart will be able to vary the conditions for particular metals so as toobtain resultant porous plate. Under varying conditions sheets wereobtained less than 0.0005 inches thick to over 0.25 inches thick.

In the case of readily oxidizable metals, e.g., aluminum, the largesurface area in the pores readily oxidizes upon exposure to air. Byselecting a medium that dissolves the oxide but not the metal, e.g.,concentrated nitric acid for aluminum, the pores size can be enlarged.For metals not so readily oxidizable, the pore size can be adjusted,i.e., decreased by heating the sheet while passing air or oxygen throughit in order to oxidize the surfaces of the pores.

Several variations can be made within the scope of the present inventionby those skilled in the art once given the benefit of the presentdisclosure. Accordingly it is intended that the foregoing specificationand accompanying drawings shall be read as illustrative and not in alimiting sense.

What is claimed is:

1. The method of producing a porous sheet structure usable as a fluidpermeable, porous plate element in filtering, sorption, reverse osmosisback-up plates, fuel cell electrode and like applications, comprisingthe steps of:

a. placing a substrate and a metal in a vacuum chamber having a pressurebetween 1 X 10' mm. Hg. and l X 10' mm. Hg, the metal being a materialwhich is capable of being vacuum evaporated and the substrate having awide area smooth surface, the metal and substrate being placed inopposing relationship in the vacuum chamber and spaced from each othertherein;

b. heating the metal to evaporate it to cause the vapor so formed totravel to the substrate and condense thereon and heating the substrateat a temperature selected as too high for powdery coating formation andtoo low for equiaxed grain formation in the coating formed by thecondensing vapors on said substrate surface,

c. the coating and condensation conditions being controlled to produce asheet-form coating of at least 0.0005 inches thick and no greater than0.02

steps of:

d. controllably oxidizing the metal at the crevass-like gaps andleaching said oxide to adjust pore size to a desired value and therebyadjust permeability of the sheet.

3. The method of claim 1 further comprising the step of stripping thecoating from the substrate.

1. The method of producing a porous sheet structure usable as a fluid -permeable, porous plate element in filtering, sorption, reverse -osmosis back-up plates, fuel cell electrode and like applications,comprising the steps of: a. placing a substrate and a metal in a vacuumchamber having a pressure between 1 X 10 4 mm. Hg. and 1 X 10 2 mm. Hg,the metal being a material which is capable of being vacuum evaporatedand the substrate having a wide area smooth surface, the metal andsubstrate being placed in opposing relationship in the vacuum chamberand spaced from each other therein; b. heating the metal to evaporate itto cause the vapor so formed to travel to the substrate and condensethereon and heating the substrate at a temperature selected as too highfor powdery coating formation and too low for equiaxed grain formationin the coating formed by the condensing vapors on said substratesurface, c. the coating and condensation conditions being controlled toproduce a sheet-form coating of at least 0.0005 inches thick and nogreater than 0.02 inches thick, the sheet-form coating structure havinga columnar structure with pores in the form of elongated crevass-likegaps at the grain boundaries, with a maximum width of such gap of nogreater than 0.1 microns.
 2. The method of claim 1 comprising thefurther steps of: d. controllably oxidizing the metal at thecrevass-like gaps and leaching said oxide to adjust pore size to adesired value and thereby adjust permeability of the sheet.
 3. Themethod of claim 1 further comprising the step of stripping the coatingfrom the substrate.